Manual calibration of audiometers is well known--see the referenced article in "Handbook of Medical Technology", by Fiedler/Wassenberg, 2nd Supplement, February 1984, II-2.1.11 attachment, pages 5-7. Such calibration is time-consuming and costly. Electroacoustic measuring apparatus can also be calibrated automatically, but such automatic calibration systems have the disadvantage that calibration values which are derived are appropriate only for the then existing test; upon disconnection, the audiometer, after termination of a test, or due to loss of operating voltage for whatever reason--failure of a battery or the like for example--the audiometer must be recalibrated.